كتاب الإيمان
1
The Book of Faith
(68)
Chapter: Being kind to one for whose faith there is concern because it is weak; Prohibition of attributing Faith to someone without definitive evidence
(68)
باب تَأَلُّفِ قَلْبِ مَنْ يَخَافُ عَلَى إِيمَانِهِ لِضَعْفِهِ وَالنَّهْيِ عَنِ الْقَطْعِ بِالإِيمَانِ مِنْ غَيْرِ دَلِيلٍ قَاطِعٍ

Sa'd narrated it on the authority of his father (Abi Waqqas) that he observed:

The Messenger of Allah (ﷺ) distributed shares (of booty among his Companions). I said: Messenger of Allah! Give it to so and so, for verily he is a believer. Upon this the Messenger of Allah remarked: Or a Muslim. I (the narrator) repeated it (the word" believer" ) thrice and he (the Holy Prophet) turned his back upon me (and substituted the word)" Muslim," and then observed: I bestow it (this share) to a man out of apprehension lest Allah should throw him prostrate into the fire (of Hell) whereas in fact the other man is dearer to me than he.
حَدَّثَنَا ابْنُ أَبِي عُمَرَ، حَدَّثَنَا سُفْيَانُ، عَنِ الزُّهْرِيِّ، عَنْ عَامِرِ بْنِ سَعْدٍ، عَنْ أَبِيهِ، قَالَ قَسَمَ رَسُولُ اللَّهِ صلى الله عليه وسلم قَسْمًا فَقُلْتُ يَا رَسُولَ اللَّهِ أَعْطِ فُلاَنًا فَإِنَّهُ مُؤْمِنٌ ‏.‏ فَقَالَ النَّبِيُّ صلى الله عليه وسلم ‏"‏ أَوْ مُسْلِمٌ ‏"‏ أَقُولُهَا ثَلاَثًا ‏.‏ وَيُرَدِّدُهَا عَلَىَّ ثَلاَثًا ‏"‏ أَوْ مُسْلِمٌ ‏"‏ ثُمَّ قَالَ ‏"‏ إِنِّي لأُعْطِي الرَّجُلَ وَغَيْرُهُ أَحَبُّ إِلَىَّ مِنْهُ مَخَافَةَ أَنْ يَكُبَّهُ اللَّهُ فِي النَّارِ ‏"‏ ‏.‏
Reference : Sahih Muslim 150 a
In-book reference : Book 1, Hadith 285
USC-MSA web (English) reference : Book 1, Hadith 276
  (deprecated numbering scheme)